A(1) Field of the Invention
The invention relates to a method of digitizing a time-discrete video signal using a picture transform coding and an adaptive variable word length coding, this time-discrete video signal being converted into a digital signal whose components comprise unequal numbers of bits.
The invention also relates to an arrangement for carrying out this method.
A(2) Description of the Prior Art
Over the years there has been an increasing interest in digitizing analog information signals. To that end an analog-to-digital converter is usually used which converts an analog information signal into a digital signal consisting of a sequence of components or code words, each comprising a number of bits. Embodiments of analog-to-digital converters are described in reference 1 (see paragraph D). The digital signal produced by an analog-to-digital converter, and consisting of a sequence of components or code words each comprising a number of bits may also be considered as a data signal consisting of a sequence of bits having either the value "0" or the value "1". Compared with analog signals, such data signal have the considerable advantage that noise superimposed on them during transmission can be removed, as a result of the fact that the bits of such data signals can be regenerated. This accomplishes a considerable improvement of the signal-to-noise ratio. For these reasons speech signals have already been digitized for quite some time in telephony systems before they are transmitted, particularly when this transmission is to be effected over long distances.
When video signals have to be transmitted over long distances, or when they have to be stored in a storage medium, it would also be advantageous to digitize these video signals. A disadvantage of a digital video signal is the enormous number of bits required to represent one full TV-picture. For a straight-forwardly digitized colour television signal this number is approximately 5.multidot.10.sup.6 bits.
If such a digital video signal were transmitted over an existing transmission line in a telephony system it would require a bit rate of approximately 120 Megabits per second, and an excess demand would be made on the capacity of this transmission line. In this respect the CCITT (see reference 2) now dictates that a digital video signal may be transmitted over a transmission line in a telephony system only when the bit rate of this digital video signal does not exceed 34 Megabits per second. This means that the original bit rate and, consequently, also the number of bits per TV-picture must be reduced by a factor of approximately four without, of course, much loss in picture quality.
Several methods have already been described for this reduction. These methods are all based on reducing the redundancy in the video signal. These methods may be divided into two categories. The first category comprises those methods in which a sample of the video signal is not coded in its entirety, but each time only the difference between two consecutive video signal samples. This method is called differential pulse code modulation, DPCM for short. This method can be realized with comparatively simple equipment and furnishes good results, provided no greater reduction of the number of bits per TV-picture is required than a factor of three (see, for example, reference 3). The second category, with which a larger reduction of the number of bits per TV-picture can be obtained, includes methods involving the so-called picture transforms for which the TV-picture is divided into a large number of usually square subpictures, each subpicture thereafter being "series developed", that is to say considered as a sum of a number of mutually orthogonal basic pictures, each having its own weight factor. These weight factors are coded instead of the video signal samples themselves.
In practice the said subpictures are obtained by combining a number of video signal samples, which either belong all to the same video line or to different video lines, into a group which is denoted a video group hereinafter, and has a finite number of elements x(n), wherein n=1,2,3, . . . N. As mentioned before each element x(n) represents a video signal sample. Thereafter this video group is converted into a group of coefficients consisting of the N coefficients y(m), the relationship between a coefficient y(m) and the N elements x(n) of a video group being given by the expression: ##EQU1## where m=1,2,3, . . . N. In this expression h(m,n) is a constant and may be considered as an element of a N.times.N matrix H. Each coefficient y(m) represents one of the above-mentioned weight factors.
It will be clear that if all these coefficients were coded with the same degree of accuracy as the original video signal samples, so requiring the same number of bits, no reduction of the number of bits per TV-picture would be obtained. In order to realize such a bit reduction the transform matrix H is chosen in such manner that the coefficients y(m) are more independent from each other than the video signal samples x(n). The transform matrices most frequently used in this connection are the Hotelling, the Fourier, the Hadamard and the Haar matrices (see, for example, references 4 and 5).
The coefficients obtained by means of this transform can be coded in two different ways, namely non-adaptively or adaptively. A non-adaptive coding of these coefficients is described in reference 4 and is there denoted "Zonal filtering" or "Masking", in which not all the coefficients are coded and transmitted or stored, but only a fixed number of predetermined coefficients. Which particular coefficients are used is not determined by the picture itself but by "all" pictures to be coded. Extensive research has shown that the absolute value of certain coefficients is small to very small on an average, while other coefficients have an absolute value which is on an average above a given value. Those coefficients whose absolute value is on an average less than a predetermined threshold value are now never coded, while the others always are. This may be interpreted as follows: each coefficient is converted into a code word having a number of bits which is characteristic of the relevant coefficient. In other words, a number b(m) which indicates the number of bits into which the coefficient y(m) must be coded, may be assigned to each coefficient y(m). These numbers b(m) may be considered to be the elements of a group B, which will be denoted a bit assignment group hereinafter. In the non-adaptive coding considered here those numbers b(m), which are assigned to those co-efficients y(m) which must not be coded, are equal to zero. Further, in this special case, b(m) becomes smaller when the above-mentioned average value of the coefficient to which this number b(m) is assigned, reduces.
Reference 4 also describes an adaptive coding of the coefficients y(m), which is called "threshold sampling" there. Also in this case not all of the coefficients are coded, but only certain selected ones. Which coefficients must be coded and how many is now determined by the picture to be encoded. These coefficients which have an instantaneous absolute value which is greater than a predetermined threshold value are now coded. As now it is not known in advance which coefficients will be coded, information about the index m of the coded coefficient must now also be generated. Also now it may be assumed that a fixed bit-assignment group B is associated with the group of coefficients, whereby those numbers b(m) assigned to those coefficients y(m) which must not be coded now are made equal to zero.
Reference 5 describes an alternative adaptive coding of the coefficients. In this case the "picture activity" of a subpicture is determined, that is to say a quantity E is calculated which either satisfies the equation: ##EQU2## or the equation: ##EQU3## Thereafter the value of E obtained in this way is compared with a number of threshold values D(1), D(2) . . . and it is determined between which threshold values or in which interval E is located. Now, a one bit-assignment group B(j) of a number of bit assignment groups j=1,2,3 . . . is associated with each interval. More particularly, with the interval E.ltoreq.D.sub.1, for example there is associated the bit assignment group B(1), the bit-assignment group B(2) is associated with the interval D.sub.1 &lt;E.ltoreq.D.sub.2, and the bit-assignment group B(3) is associated with the interval D.sub.2 &lt;E.ltoreq.D.sub.3, etc. These bit-assignment groups B(j) are characterized in that several elements b(j,m) will have the value zero, while an element b(j,m) differing from zero need not be equal to the non-zero element b(i,m) for i unequal to j.
It should be noted that generally both a linear and a non-linear coding characteristic may be used for the coding of a coefficient y(m).